The present invention relates to a system and method for disposing of wastewater in marine vessels in an environmentally friendly manner. The present invention also relates to a system and method for maintaining a bilge of marine vessel relatively free of an accumulation of oil. The present invention further relates to a system and method for separating oil and other contaminants from wastewater, to produce relatively clean process water. The present invention further relates to a system and method for disposing of wastewater in a vessel, including bilge water.
The treatment and disposal of wastewater, and in particular, contaminated wastewater, has proven to be a difficult task to perform and enforce in practice. While this is true in the average case, it is particularly true for the treatment of contaminated wastewater produced on transportation vessels such as marine vessels.
A marine vessel, when in operation produces large quantities of wastewater that must be treated and/or stored until the vessel reaches port. The alternative is for the vessel to dispose of the wastewater overboard and into the body of water through which it is travelling.
Marine vessel wastewater can be divided into broad categories. The first results from human activity on board the vessel and includes “sewage” and “grey water”. Grey water is typically understood to be wastewater produced as a result of cooking, cleaning, washing and other personal activities. The second category of wastewater on a marine vessel is typically understood to be “bilge water” which includes liquids and particulates that accumulate in the bilge as a result of activities relating to the operation and maintenance of a vessel.
Bilge water constitutes all water that makes its way into the bilge of the vessel. Water can find its way into the bilge in a number of fashions, including water from leaking pipes, valves and pumps, water that may enter the engine room from the propeller tail shaft, water leaks from equipment located in the engine room, process water used to wash the engine room and other industrial areas of the marine vessel, turbid or brackish water drawn onboard from ports and harbours and even some grey water, all of which may find its way into the bilge. The resulting accumulation of water in the bilge is typically contaminated with lubricating oil, fuel oil and other debris from the engine room.
The engine room bilge on marine vessels serves as a catch all or dump for all of the water, lubricating or hydraulic oils, fuel oil (heavy and diesel), cleaning additives such as soap, degreasers, mop water and any other liquids or waste water that may be spilled in the engine room or machinery spaces of the vessels. Added to this water is other detritus including: dirt, dust and other particulate from the atmosphere; dust, spoils and other residues from cargo handling operations; and carbon and other residue accruing from the cleaning of machinery and equipment. It is not uncommon to find cleaning rags, filings, paint chips, rust and scale in a vessel's bilge. The resulting mixture may be called “bilge water”, a particularly contaminated form of industrial wastewater.
Marine wastewater accumulating in the bilge has different characteristics depending upon its contents at any given time. Typically, in periods when there is a significant accumulation of water and contamination within the bilge, oil will float on top of the water surface as it has a lighter specific gravity. Mixed with the oil on top of the water will be lighter solids, soaps, degreasers, and other chemicals to form an emulsified mixture within the top layer of water. The remaining portion of the bilge water below the surface is typically water and any heavier solids. Typically, the heavier solids and particulates do not remain in suspension and settle out to the bottom of the bilge.
Accordingly, the contents of the bilge may be generalised as consisting of three layers: an “upper layer” comprised mostly of oils and other liquids having a specific gravity lighter than water; a “bottom layer” comprised mostly of solids and sediment consisting of saturated particulates; and a “middle layer” comprised mostly of water and fine particulates held in suspension. In relative terms the “middle layer” is often referred to as the “clean water layer”, as in comparison to the other layers it comprises a smaller fraction of oil and particulate, though it is not sufficiently “clean” as to be potable or directly discharged into the environment. This generalisation is understood to be for descriptive purposes and it should be understood that the layers are not homogeneous and typically consist of varying proportions of all the contents of the bilge.
The standard practice in dealing with such contaminated water has been to simply pump, collected from the “middle layer” and below the “upper layer”, water from the bilge that is presumed to have a lower content of oil and other contaminants. This “clean water” component of the bilge is passed through oil traps such as filters or scrubbers prior to discharge over the side of the vessel.
In order to meet various environmental regulations, which until recently have focused mostly on the oil content of wastewater to be discharged overboard from marine vessels, a meter (known in the marine industry as an “oil content meter”) must, in accordance with the International Maritime Organization's requirements, be installed on the discharge line from the bilge. The purpose of the oil content meter is to ensure that any water discharged from the vessel does not contain more than the maximum amount of oil permitted to be discharged with the water, measured in parts per million. Generally, these meters have a reputation of unreliability. In practise, they merely confirm the clarity of the water to be discharged and may be defeated by diluting the concentration of the wastewater with additional water from other sources.
In practical terms, oil content meters have been found to be unreliable for two reasons. First, they are subject to fouling due to normal operating conditions onboard the vessel and periodically malfunction as a result. Second, the meters are often incapable of distinguishing the various contaminants that may be present in the water. The typical oil content meter triggers an alarm based on a reading of the turbidity of the water to be discharged. If the reading is high, the meter presumes that the water to be discharged contains oil in an amount that exceeds permissible levels and discharge operations are stopped, the water is directed back to the bilge and an alarm signals the crew. However, contaminants other than oil, such as turbid or brackish water drawn onboard in the course of normal operations, cleaning fluids, soaps, dust, dirt, rust, salts, dissolved minerals and other particulates also affect turbidity. Often, the presence of “non oil contaminants” in the discharge line causes the oil content meter to trigger a “false reading” with respect to oil content in the water and discharge operations are stopped as a result.
To overcome these operational deficiencies, the practical reality is that many vessels attempt to defeat the operation of the oil content meter. Although it is illegal to do so, there several ways to circumvent the oil content meter. One is simply to divert water through another line that bypasses the meter. Another is by allowing or injecting additional water into the discharge stream, up-stream from the meter. The effect of the introduction or injection of additional water into the discharge stream is to reduce the concentration of any contaminants in the wastewater. Further, if the oil content meter malfunctions, there is no other automated system onboard that prevents the discharge of contaminated water overboard. As a result, the monitored restriction and management of the overboard discharge of wastewater, based on the use of oil content meters, has proven largely ineffective at ensuring compliance with environmental regulations.
An additional problem with these existing systems is that by allowing for continual discharge of wastewater over the side of a vessel, the bilge effectively becomes a repository of higher and higher concentrations of oil and other contaminants. This is as a result of consistently siphoning off the “clean water layer” from the bilge, thus allowing the oil and other contaminants on the “upper layer” of the bilge water to remain and expand. The net result is a bilge that grows dirtier and dirtier with time and comprises an ever increasing proportion of oil in the bilge.
Environmental regulations have been enacted around the world in recognition of the fact that marine wastewater is particularly harmful to the environment.
In prior art systems for dealing with bilge water, the system typically relies on elements such as a decant tank, as well as, polishing media such as filters and other scrubbers to remove the oil content from the bilge water that is siphoned from the bilge. While such media can be effective at removing bulk quantities of oil from wastewater, they suffer from several disadvantages within the context of a continually operating system.
First, media such as filters and scrubbers become clogged with time and require replacement or cleaning. For applications such as marine vessels, often there may not be spare parts available for replacement when a ship arrives at port. In the result, the system may be rendered inoperable or ineffective. Further, the spent filters and scrubbers must be stored onboard until the vessel reaches port and, in themselves, constitute hazardous or contaminated waste materials dangerous to the environment. Second, a filter or scrubber may break while the vessel is in transit, resulting in the wastewater system being unable to remove oil and contaminants from bilge water prior to discharge over the side. The system may be rendered inoperable or ineffective until the vessel reaches port, assuming that replacement parts are available and can be installed in a timely fashion. Third, while such systems are capable of removing oil from bilge water by trapping and holding it in a filter, they are of marginal utility for the purpose of capturing, recovering and storing oil for re-cycled use. Fourth, such systems typically remove water from the vessel's bilge, filter it and discharge it off the vessel, the sole practical purpose being to restore the holding capacity of the bilge. The bilge water removed is neither treated nor re-cycled for any other operational use onboard the vessel. While these shortcomings have been recognized for many years, to date there has been no practical solution for dealing with wastewater from the bilges.
Referring to FIG. 1a, a prior art method of handling bilge water is depicted. A bilge 100 may be generally described as a catch-all for wastewater free in a vessel. As a result of the catch-all nature, a bilge 100 typically comprises a bottom layer of heavy particles and sediment 505, a middle layer of water and fine particulate 510 and an upper layer of oil 515. In prior art bilge systems a bilge water intake 21 is typically provided just above (such as 6″) the bottom layer 505 of the bilge 100. The intake 21 is located to minimize the intake of sediment from the bottom layer 505 of the bilge 100, and avoid drawing oily water from the upper layer 515 of the bilge 100.
A hi-level bilge sensor 22 and low-level bilge sensor 23 are provided to control the operation of a pump 205. The pump 205 draws bilge water from the bilge 100 through bilge water intake 21, and directs it through an oil trap 24 before discharging the bilge water overboard. Water separated in the oil trap may be diverted to an oil store 26 such as a sludge tank.
Prior bilge wastewater handling systems of this type generally relied upon an oil trap 24 that operated by gravitational settlement or filtration. Since these methods take time and are not well-suited to operating on large volumes of liquid, the bilge water intake 21 was necessarily located below the low-level sensor 23 in an attempt to draw water from only the middle layer 510 in the bilge 100 and avoid drawing oily water from the upper layer 515. Accordingly, the pump 205 is only energized when a sufficient amount of water has collected in the bilge 100 to locate the intake 21 in the middle layer 510.
In order to comply with environmental regulations, a detector 25 monitors the condition of the water exiting the oil trap 24. Typically, the detector 25 detects a level of turbidity in the water leaving the oil trap 24 to ensure the concentration of oil in the water is below acceptable limits. If the turbidity is too high, the water is directed back into the bilge 100 and an alarm condition is initiated to indicate that maintenance of the oil trap 24 is required.
Common problems with this arrangement are that other contaminants in the water, such as soap, sediment or other particulate will also trip the detector 25 even when the level of oil is within acceptable limits. As a result it is a common practice (though illegal) in the shipping industry to operate the bilge discharge by adding additional fresh water to the bilge discharge in order to maintain a low turbidity discharge through the detector 25.
Typical oil traps 24 consist of absorbent material, specific gravity traps, or other filters to remove oil from the bilge water extracted from the bilge 100. Since, the oil traps 24 commonly used are unable to handle large amounts of oil, preferably the intake 21 extracts liquid only from the middle layer 510, for discharge overboard. Referring to FIGS. 2a-2c an illustration of the effect of removing liquid from the middle layer 510 is illustrated. FIG. 2a illustrates a level of the bilge and the three layers described above. In FIG. 2b, the level of the bilge has risen above the high-level sensor 22 and pumping is performed. Over time and multiple pumping cycles, FIG. 2c results with a build-up of oil in the upper layer 515 in the bilge 100. After repeated cycles, the level of oil in the upper layer 515 has built up to such an extent that the oil in the upper layer 515 is close to the intake 21 when the liquid surface 516 is above the low-level sensor 23 and near the high-level sensor 22, and the ship must put into shore for clean-up of the bilge 100.
The build-up of oil in this manner is dangerous and results in a bilge 100 that is an environmental and personal hazard to the vessel's crew. Due to the oil build-up over time clean-up is generally time consuming, and results in the vessel being held up in port for maintenance. If the build-up escapes from the bilge 100 it may cause damage to the environment and depending on the laws and regulations in the jurisdiction having authority, expose the vessel's operators and/or owners to significant penalties including environmental fines, civil claims for damages, criminal convictions and/or incarceration.
Among other problems with this system, the retention of oil leads to a messy bilge environment that can pose both a fire and an environmental risk. This condition also makes it difficult to detect and repair “fresh” leaks of oil, fuel and lubricants from equipment located in the engine room. Furthermore, recent proposed environmental legislation reflects a developing “zero tolerance” policy regarding discharges not only of bilge water from the bilge 100, but also of wastewater of any description from other sources onboard including effluent, grey water and blow down of water containing chemicals. Meeting this standard is not possible using current techniques.
FIG. 1b is a prior art proposed solution for disposing of vessel bilge water. Bilge water is extracted from below the surface of the liquid at an intake 21 and passed through an oil trap 24 and the oily waste is directed to oil storage 26. The treated bilge water is then either discharged overboard or injected (sprayed) at an injection point 32 inside an exhaust stack 31 of the vessel's engine 30 directly into the exhaust stream 33 for flash vaporisation.
Aside from the practical difficulties with injecting large amounts of water into an engine exhaust stack 31, the solution suffers from the drawback that water may only be vaporised when the vessel is operating at full throttle, with the engine 30 at full pressure and with an exhaust stack 31 temperature over 500° F. Accordingly the solution is not practical for zero-discharge operation: while in stationary or in port, while the vessel is in transit at less than full throttle, or for disposing of large quantities of bilge water.
In practical terms, the solution also suffers from other significant operational shortcomings. First, since the solution requires extreme temperatures to achieve flash vaporization, the rate of water that may be injected into the stack 31 is limited as the injection of cold water into the exhaust stack 31 reduces the ambient temperature of the exhaust gases 33. Second, especially if the ambient temperature falls below 500° F., there is a danger that any particulate introduced into the stack 31 and any steam generated by the injection process may not be fully exhausted up and out the stack 31. If this happens, gravity will cause the residue to fall into the engine 30. Third, the resulting increased humidity in the exhaust gases 33 may lead to increased corrosion of the stack 31.
There thus arises a need for a system and method for handling wastewater from bilges that overcomes the limitations of the prior art. There also arises a need for a system and method that is capable of handling wastewater from bilges that can operate in the current climate of ever increasing environmental regulations. There further arises a need for a wastewater management handling system and method that can achieve zero discharge into the bodies of water on which the vessel travels.